1. Field of the Invention
The invention relates generally to methods for making precise non-contact temperature measurements of a sample based on its thermally-emitted radiation, as well as methods for calibrating such a temperature measurement device.
2. Related Art
Many advanced manufacturing processes depend on the ability to control the temperature of a sample with high precision and repeatability. However, use of contact-based measurement techniques is often impractical. As one example, thin film and semiconductor deposition processes used for manufacturing electronic and optical devices require very precise control of substrate temperatures to yield high quality products, but the temperature measurements must be taken with equipment located entirely outside of the deposition chamber. As another example, molecular beam epitaxy (MBE) is used as a method to precisely deposit ultra-pure elements like gallium or arsenic in many advanced electronic applications. It is not feasible to use contact-based thermometry in these situations either, even though the quality of the manufacturing process depends upon temperatures being held to tight tolerances. Indeed, many other applications exist in which precise real-time temperature monitoring and/or controls are needed, but must be accomplished with non-contact measuring instruments.
Several methods for non-contact temperature monitoring have been proposed. One very common method is based on a form of pyrometry, for example, in which radiation intensity from the sample is detected in a specific wavelength range. Using known mathematical equations like Planck's law, the detected radiation intensity can be correlated to sample temperature via a fundamental relationship between the emission signal and the sample temperature. For a given temperature, these emission characteristics over all wavelengths (that is over the blackbody spectrum—BB) obey Planck's law. Planck's law states that the spectral radiance at any wavelength for a given temperature conforms to the mathematical equation:
      I    ⁡          (              λ        ,        T            )        =            A      ⁢                        2          ⁢                      hc            2                                    λ          5                    ⁢              1                              ⅇ                                          hc                /                λ                            ⁢                                                          ⁢                              k                B                            ⁢              T                                -          1                      +    C  Where:                I(λ,T)=spectral radiance as a function of wavelength and temperature        T=the temperature in Kelvin        λ=the wavelength        A=the amplitude variable which is a product of the emissivity of the sample and a tooling factor        h=Planck's constant        c=the speed of light        kB=Boltzmann's constant        e=the base of the natural logarithm and        C=a constant offset or background.        
When plotted, this mathematical equation yields a predictable emission intensity curve versus wavelength for any given temperature. This relationship forms the basis of standard pyrometry. Thus, standard pyrometry relies on measuring the integrated light intensity for a fixed time period over a narrow wavelength range. Standard pyrometry approaches assume that the sample emissivity will not change over time, that the sample is opaque at the measurement wavelength, and that no other intensity contributions or attenuations exist from non-sample related sources (e.g., from viewport coatings, stray filament radiation, and the like).
Such standard pyrometry techniques prove inadequate to address many of today's highly sophisticated manufacturing processes that require consideration for stray light, emissivity changes during deposition, viewport coating buildups, and the like. Also, the standard pyrometry techniques require some type of in-situ calibration on a routine basis to compensate for chamber-dependent factors. When monitoring temperature for semiconductor materials, for example, it is also important to collect signal intensity information for pyrometry at a wavelength where the material is opaque, i.e., above the material's band gap or band edge.
Another non-contact thermometry technique may include monitoring the band gap absorption edge from a sample and correlating the temperature-dependent band gap to the sample temperature via empirically-derived data.
An improvement on the standard pyrometry technique, known as emissivity-correcting pyrometry (ECP), may be seen by reference to U.S. Pat. No. 5,377,126 to Flik et al., issued Dec. 27, 1994. The Flik patent incorporates a methodology by which changes in sample surface emissivity are factored into the temperature determination equations.
A still further type of non-contact temperature determination, distinct from pyrometry but somewhat related in approach, is the so-called blackbody fitting technique. This method allows non-contact measurement of a sample's temperature by acquiring wavelength-dependent spectra from the heated sample and then matching that spectra to a blackbody radiation curve calculated by a mathematical equation based on Planck's law (as given above). The spectral radiance is the fundamental measure of the amount of light emitted from a diffuse source that can reach a spectrometer or other suitable detector. Spectral radiance is defined as the emitted power per unit area of emitting surface, per unit solid angle, per unit wavelength (mW cm−2 nm−1 sr−1). For example, FIG. 3 describes a plot of the spectral radiance of an ideal blackbody for several temperatures. Note that every spectral radiance curve is unique for a given temperature, i.e., the curves defined by Planck's mathematical equation never cross. Furthermore, the peak of each curve shifts toward shorter wavelengths as the temperature increases. The wavelength of the peak radiance obeys Wien's displacement law:
      λ    max    =      b    T  where the constant b=2.8978×106 nm K. Note that in the temperature range for many advanced manufacturing processes, the peak of the radiance curve lies in the mid-IR (infrared) portion of the spectrum. Thus, using standard spectrometer technology, most temperature measurements are based on fitting to the short-wavelength exponential tail below the peak. This is exemplified in FIG. 3 where the broken vertical lines between 900 and 1,700 nm represent an approximate detection range for a standard solid state spectrometer. Thus, it is within this range that spectra data are acquired and analyzed by the blackbody fitting technique to determine the temperature of a sample.
An early example of the blackbody fitting technique may be seen by reference to U.S. Pat. No. 5,132,922 to Khan et al., issued Jul. 21, 1992. The entire disclosure of U.S. Pat. No. 5,132,922 is hereby incorporated by reference. This patent performs either a non-linear (in the case of the full Planck distribution function) or a linear (in the case of the Wien approximation) least squares fit of the acquired spectral data to the appropriate function. By design, this technique requires an estimate or functional form of the emissivity versus wavelength dependence for whatever blackbody/graybody is being measured. A particular drawback in the concept described in this patent is its functional dependence on the emissivity of the sample being measured. This dependence on emissivity has the practical effect of introducing measurement errors. Such errors arising from the use of estimated wavelength and temperature-dependent emissivity are noted in U.S. Pat. No. 6,738,724 to Macintosh, issued May 18, 2004. The entire disclosure of U.S. Pat. No. 6,738,724 is hereby incorporated by reference. U.S. Pat. No. 6,738,724, in turn, requires that different surface areas and spectral regions of the radiation source be imaged onto different spectrometer channels via computer-controlled auto-focus and auto-zoom optical elements, and via waveguide cables. This is an exceedingly complicated approach which is cumbersome and likely to introduce substantial errors in the temperature measurement process.
This, as with the other prior art systems, creates difficulties in real world applications of the technology. One such shortcoming is a general failure to adequately address variations in spectrometer response with wavelength. That is, prior art systems fail to adequately correct for variations in the spectrometer that may be present across the entire spectra such as, for example, due to the inherent response of the optical components. Another shortcoming inherent in these prior art approaches to temperature measurement arises out of the necessity to first calibrate the measurement system to some external reference before actual temperature measurements can be made. Such external references might derive from a band edge measurement or RHEED transition, or by other suitable thermometry techniques. Thus, the pyrometry and blackbody fitting techniques of the prior art are not self-sufficient as a means for determining sample temperature, but rather must be paired with another type of measurement system for at least an initial calibration phase. This, of course, increases the cost and complexity of manufacturing processes that need thermometry data for proper operation. Also, this initial calibration step usually requires elevating the sample temperature to a high level, introducing time delay and reducing efficiencies in the manufacturing processes.
Accordingly, there is a need in the art for improved techniques of non-contact temperature measurements using blackbody fitting methods which do not suffer from the aforementioned spectral response variations, and further, which are not dependent upon initial calibration using a supplemental measurement technique.